Forest Patch Geometry and Climate Regulate the Impact of Forest Thinning on Snowpack in the Southwest United States

Despite having important implications for water resources, the climatic dependence of forest thinning impacts on snowpack is poorly quantified. In this study, we used a high-resolution snow model to understand the impact of forest thinning on snowpack in Arizona under contrasting climate conditions, leading to ephemeral vs. seasonal snowpack conditions. The model is evaluated using a spatiotemporally extensive set of snowpack measurements and is run for the same set of pre- and post-thinning forest patch geometry using two meteorological forcing datasets representing locally mid- and high-elevation climate conditions. Although the high-elevation climate is only 1°C cooler and has 20% more winter precipitation, it leads to markedly different snowpack conditions, i.e., twice as long-lasting snowpack, less mid-winter ablation events and ~60% larger at its peak. For both climates, forest thinning increased peak snow water equivalent (SWE) and liquid water input (LWI), but it decreased snow cover duration (SCD) only for the high-elevation climate. Total sublimation losses decreased from ~35% of wintertime precipitation pre-thinning to ~25% post-thinning for the high-elevation climate and from ~25% to ~15% for the mid-elevation climate. Generally, a 10% reduction in canopy cover resulted in ~4.5% more snowfall reaching the ground, and a 10-day decrease in SCD reduced the fraction of winter precipitation lost to snowpack sublimation by ~2%. Post-thinning changes in forest patch geometry were also important as larger canopy gaps had more LWI, and areas with warmer canopy edges had lower peak SWE and SCD.